1. Field of the Invention
This invention relates to fluid-foils and, more particularly, to aerodynamic means at the tips thereof for controlling and reducing tip vortices to thereby decrease lift-induced drag.
2. Description of the Prior Art
As is well-known, fluid foils, are shaped or positioned such that, when they have relative motion with respect to a fluid in which they are immersed, the fluid travels faster over the upper surface than over the lower surface. This produces a lower pressure on the upper surface that generates lift. An airfoil, such as an aircraft wing of finite length, has a tip end presenting a path for fluid from the higher pressure region on the bottom surface of the wing to flow toward the lower pressure region on the top surface in a relatively stable vortex flow. Such vortex flow degrades flight efficiency by reducing lift while increasing drag on the aircraft. Further, the wing and wing tip flow pattern establishes a pair of trailing vortices which remain undissipated for extended periods of time, posing serious flight hazard for following aircraft and for aircraft crossing the region of such vortices. Typically, a time spacing of three minutes or more is provided between take-offs and landings of aircraft to permit dissipation of the vortices, thereby contributing to delays in airport operations. If the airfoil is used as the blade or the wing of a rotary-wing aircraft, substantial loading and rotor noise are generated as the blade passes through the vortices created by a preceding blade.
Prior attempts to overcome, deflect, or dissipate the adverse effects of wing tip vortices have been made, but have been subject to disadvantages and limitations. Several prior art constructions for the purpose are disclosed by A. W. Loerke, U.S. Pat. No. 2,075,817. Each Loerke construction has a disadvantage or limitation; however, such as a requirement for a venturi tube open at the wing tip, creating air resistance in flight; or a requirement for a suction pump to draw air from the wing tip and through the wing; or for a requirement for an external structure appended at the trailing edge of each wing tip. The requirement for a drag-inducing pod-like body at each wing tip is also a disadvantage in the constructions disclosed by W. R. Haney, Jr., in U.S. Pat. No. 3,596,854. The complexity and added cost of the jet engines used to control tip vortices militates against the benefits of the design disclosed by J. R. Erwin, U.S. Pat. No. 3,997,132.
Investigators have also explored the possibility of reducing drag below that given by optimum wing sections (minimum friction and wave drag) and an elliptic span load (minimum conventional lift-induced drag) by using laminar flow control (LFC) and various types of winglets. The implementation of these technologies is hindered by many practical considerations. The anticipated 25% friction drag reduction attributed to LFC may not be sufficient to offset the power requirements, weight penalties, and the problems of system maintenance occasioned by the use of the technique. Similarly, the 4-10% winglet drag reduction, which is very configuration dependent, is somewhat compromised by the added drag and structural weight penalties of the winglets themselves. As a result, industry acceptance of these technologies has been slow.
Control of the wing tip flow has also been effected by shaping the wing tip to produce a simple end plate effect. Such a down-turned wing tip has the effect of inhibiting or altering the lower surface crossflow of the wing. This, in turn, weakens or modifies the wing tip vortex which effects the induced drag created by the wing downwash field. A design of this type taught by W. E. Sargent in U.S. Pat. No. 3,411,738 was incorporated in a Fairchild A-10 aircraft and resulted in a measure of drag reduction; however, it is seen that a drooped wing tip involves wing tip vortex deflection rather than a cross-flow capture and removal which is the principle by which the subject invention operates.
A drooped-tip wing which does have a provision therein for crossflow capture and removal is disclosed by E. H. Johnstone, U.S. Pat. No. 3,974,986. It has been found, however, that the design of the wing tip capture surface is critical for the successful operation of the concept. With the wing tip designs taught by Sargent and Johnstone, experiments have shown that while a small end-plating effect was observed, the suction or flow removal was substantially negligible. In addition, the design disclosed by Johnstone requires modifications to the structure of the wing itself. Since the cost and weight penalties required by such modifications are high, the practicality of the design suffers thereby.